1. Field of the Invention
The present invention relates to an image scanning device included in an image forming apparatus such as a copier, a facsimile machine, and a printer.
2. Description of the Related Art
Conventionally, in image scanning devices, light is irradiated to a white standard plate disposed opposite to a scanning unit, and the light reflected from the white standard plate is received to acquire shading data indicating the white density at the time of scanning an original. Based on this shading data, shading correction is performed. In performing shading correction, it is important that the corrected values be uniform in the width direction of the original. Accordingly, it is preferable that the white standard plate be highly flat so that the distance from the scanning unit to the standard white plate varies within a predetermined range. There are white standard plates that are fabricated by performing press work on sheet metal and adhering white seals or applying a white coating on their surfaces, which white standard plates are low-cost and widely used.
However, in an image scanning device that is wide enough to accommodate the width of an A0-sized sheet (the width of an A0-sized sheet is 841 mm), the white standard plate, which is formed by performing press work on sheet metal, may sag in its longitudinal direction. Thus, it is difficult to maintain the high flatness of the white standard plate. The white standard plate needs to have high rigidity in order to increase its flatness, so that the white standard plate is prevented from sagging as much as possible.
A method of fabricating a highly rigid white standard plate is disclosed in patent document 1. Specifically, a member made of aluminum alloy, which is extruded from a die, is used to form a white standard plate having a box-like shape that has high rigidity.
Patent Document 1: Japanese Laid-Open Patent Application No. 2005-159482
However, in order to fabricate a white standard plate by extruding aluminum alloy, expensive materials and costly processes are required, which lead to increased costs. Furthermore, the board may become curved or twisted, which are typical features of extrusion processing. Hence, additional processes are necessary for correcting such deformations, which lead to even higher costs.